Hitherto, in golf balls having a multilayer construction of which core is covered with a cover, an ionomer resin has been widely used as a resin for the cover (e.g. Japanese Patent Laid Open Publication Nos. 149277/1990, 182279/1990, 88087/1990, 107275/1990, 308577/1989, 223103/1989, 207343/1989, 70086/1989, 229077/1988, 311973/1988, 281145/1986, 82768/1986, 212406/1985, 14879/1985, 119766/1982, etc.).
The ionomer resin is that obtained by subjecting an ionic ethylene copolymer composed of three components of .alpha., .beta.-olefin, .alpha., .beta.-ethylenic unsaturated carboxylic acid, and .alpha., .beta.-ethylenic unsaturated carboxylic acid metal salt, or four components of .alpha.-olefin, .alpha., .beta.-ethylenic unsaturated carboxylic acid, .alpha., .beta.-ethylenic unsaturated carboxylic acid metal salt, and .alpha., .beta.-ethylenic unsaturated carboxylic acid ester to partial metal crosslinking. Because of its high impact resilience as well as excellent impact resistance and cut resistance, the ionomer resin has been widely used as the resin for the cover of golf balls having a multilayer construction, as described above, and now predominates over others.
Various ionomer resins have been reported and they may be normally produced in such a manner that, using ethylene as the .alpha.-olefin and using monobasic unsaturated carboxylic acid (e.g. acrylic acid, mathacrylic acid, etc.) or dibasic unsaturated carboxylic acid (e.g. maleic acid, fumaric acid, etc.) as .alpha., .beta.-ethylenic unsaturated carboxylic acid, these carboxylic acids are partially neutralized with monovalent metal ions [e.g. sodium (Na), lithium (Li), potassium (K), etc.] or divalent metal ions [e.g. zinc (Zn), magnesium (Mg), Copper (Cu), etc.] For example, this type of the resin is commercially available under trade name of SURLYN from Du Pont de Nemours & Co. and of HI-MILAN from Mitsui Du Pont Polychemicals Co.
As described above, the ionomer resin has high impact resilience as well as excellent impact resistance and cut resistance and now dominates in the resin for the cover, and golf balls wherein the ionomer resin is used for the cover have excellent properties such as high impact resilience, etc.
However, golf balls having higer impact resilience and more excellent flying performance are requested by users.